Add presence heartbeat liveness guard

[zergzorg]

/claim #12

Summary

• Adds collab-presence-heartbeat-liveness-guard, a focused Real-Time Collaborative Editor slice for live cursor and presence fanout safety.
• Evaluates synthetic collaboration rooms before broadcast, detecting stale heartbeats, ghost sessions, reconnect-grace sessions, duplicate tabs, orphaned cursors, expired typing indicators, expired presence fanout leases, and heartbeat clock skew.
• Emits sanitized presence snapshots, cleanup queues, recovery queues, fanout events, deterministic JSON/Markdown/SVG reports, and a short H.264 MP4 demo artifact.

Why this solves part of #12

Issue #12 asks for Google Docs-style multi-user editing, live cursor tracking, user presence indicators, and activity status. Those features need a liveness gate before every presence fanout; otherwise stale tabs and broken reconnects can leave ghost collaborators, old cursors, or expired typing state visible to active authors.

This PR adds that pre-broadcast liveness layer without implementing editor UI, transport, privacy redaction, notebook execution, or section-lock recovery.

Sponsor decision packet

• What the bounty poster gets: a complete room-level presence cleanup packet with publish/suppress decisions, cleanup and reconnect queues, fanout-ready snapshots, and stable audit digests.
• How to verify it: run the listed Node checks and inspect reports/presence-liveness-packet.json, reports/presence-liveness-report.md, reports/summary.svg, and reports/demo.mp4.
• Expected synthetic result: 2 rooms reviewed, 8 sessions reviewed, 3 sessions published, 5 sessions suppressed, 7 cleanup actions, 1 reconnect recovery action, and 3 presence fanout events.

Non-overlap audit

This is separate from the existing #12 work on broad editor foundations, operation replay, offline conflict, notebook workbench and kernel leases, reference formatting and merging, authorship/submission governance, freeze/lock/checkpoint recovery, review lanes, discussion/sidebar audit, autosave recovery, round-trip fidelity, review decision, task dependency, equation/figure anchors, presence privacy, accessibility parity, evidence binding, embargo release, notification visibility, and data availability.

Scope is limited to heartbeat-based liveness and cleanup immediately before cursor/presence snapshots are broadcast.

Scope boundaries

• Does not implement production editor UI, CRDT/OT operations, section locks, notebook execution, privacy redaction, notification fanout, release/export guards, storage, or realtime network transport.
• Uses synthetic fixtures only: no live users, private manuscripts, identity data, credentials, external services, or institutional systems.

Validation

• cd collab-presence-heartbeat-liveness-guard && npm run check
• cd collab-presence-heartbeat-liveness-guard && npm test
• cd collab-presence-heartbeat-liveness-guard && npm run demo
• ffprobe -v error -select_streams v:0 -show_entries stream=codec_name,width,height,duration,avg_frame_rate -show_entries format=size,duration -of default=noprint_wrappers=1 collab-presence-heartbeat-liveness-guard/reports/demo.mp4 -> H.264, 960x540, 5s, 15 fps
• git diff --check
• git diff --cached --check
• disclosure scan over the new module showed only expected node:crypto references

Demo and artifacts

• Short demo video: collab-presence-heartbeat-liveness-guard/reports/demo.mp4
• Review packet: collab-presence-heartbeat-liveness-guard/reports/presence-liveness-packet.json
• Reviewer report: collab-presence-heartbeat-liveness-guard/reports/presence-liveness-report.md
• Visual summary: collab-presence-heartbeat-liveness-guard/reports/summary.svg